Method and apparatus for electroplating a metal onto a substrate

ABSTRACT

Method for electroplating a metal onto a flat substrate P. Surfaces are electrically polarized for metal deposition by feeding thereto at least one first and second forward-reverse pulse current sequences. The first forward-reverse pulse current sequence includes a first forward pulse generating a first cathodic current during a first forward pulse duration t f1  and having a first forward pulse peak current i f1 , and a first reverse pulse generating a first anodic current during a first reverse pulse duration t r1  and having a first reverse pulse peak current i r1 , the second forward-reverse pulse current sequence including a second forward pulse generating a second cathodic current during a second forward pulse duration t f2  and having a second forward pulse peak current i f2 , and a second reverse pulse generating a second anodic current during a second reverse pulse duration t r2 , the second reverse pulse having a second reverse pulse peak current i r2 .

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a method and to an apparatus forelectroplating a metal, copper for example, onto a substrate. Suchmethod and apparatus may be utilized in the field of electroplating anarticle to be used as an electrical device, such as a printed circuitboard, a chip carrier, including a multichip carrier, or any othercarrier having a circuitry thereon.

2. Brief Description of the Related Art

The manufacture of such electrical devices is well-known. The processesfor their manufacture comprise a plurality of steps including the metaldeposition step for producing the circuitry thereon. These process stepsrequire metallization on the outer surface of the device as well as inthe holes or other recesses in the electrical device. For example,printed circuit boards having a plurality of circuitry layers and alsohaving a plurality of holes, namely through holes and blind holes, areto be electroplated with copper in order to generate a copper deposit,the thickness thereof being required to be as uniform as possible.Furthermore, copper deposition in the holes shall also be uniform.Especially, copper deposition is to be consistent both on the outersides of the electrical device and in its holes to avoid that the holesare not provided with a sufficient copper thickness while coppering onthe outer sides has already attained the required deposit thickness.

Pulse plating has mainly proved to be suitable to meet the aboveobjectives. More specifically, reverse pulse plating has been identifiedto be particularly appropriate. Reverse pulse plating refers to aprocess comprising applying cathodic and anodic current pulsesalternately to the electrical device.

U.S. Pat. No. 6,524,461 B2 for example teaches a method for depositing acontinuous layer of a metal onto a substrate having small recesses inits surface. This method comprises applying a modulated reversingelectric current comprising pulses that are cathodic with respect tosaid substrate and pulses that are anodic with respect to saidsubstrate, wherein the on-time of said cathodic pulses is from about0.83 μs to about 50 ms and the on-time of said anodic pulses is greaterthan the on-time of said cathodic pulses and ranges from about 42 μs toabout 99 ms. In a typical example of the modulated reversing electriccurrent sequence a waveform is used which comprises a cathodic (forward)pulse followed by an anodic (reverse) pulse. An off-period of relaxationperiod may follow either or both of the cathodic and anodic pulses.

Further, US 2006/0151328 A1 teaches a method of applying a pulse reversecurrent flow to work pieces having high-aspect ratio holes, i.e., holeswhose length is large as compared to the diameter thereof. Holes havingan aspect ratio of up to 10:1 and a hole length of 3 mm or even largershall be processed efficiently. The pulse plating sequence to be appliedto the work pieces is taught to comprise cathodic and anodic pulses andis used at a frequency of at most about 6 Hertz. Durations of theforward current pulses and reverse current pulses are indicated to be atleast 100 ms (forward) or at least 0.5 ms (reverse), respectively. Thepeak current density of the forward current pulses is furthermoreindicated to be at least 3 A/dm2 and at most 15 A/dm2 and that of thereverse current pulses to be at least 10 A/dm2 and at most 60 A/dm2. Ina preferred embodiment of the process described in this document thework pieces are plate-shaped, such as printed circuit boards or anyother plate-shaped electrical circuit carriers. In this preferredembodiment, the method comprises (a) applying a voltage to between afirst side of the work piece and at least one first anode, to the effectthat a first pulse reverse current flow is provided to the first side ofthe work piece, wherein said first pulse reverse current flow has atleast one first forward current pulse and at least one first reversecurrent pulse flowing in each cycle time, and (b) applying a secondvoltage to between a second side of the work piece and at least onesecond anode, to the effect that a second pulse reverse current flow isprovided to the second side of the work piece, wherein said second pulsereverse current flow has at least one second forward current pulse andat least one second reverse current pulse flowing in each cycle time. Ina particularly preferred embodiment the first forward and reversecurrent pulses of one cycle are offset relative to the second forwardand reverse current pulses of one cycle, respectively. This offset mayadvantageously be approximately 180°. It is furthermore indicated that,for further improving throwing power, the current flow may comprise, ineach cycle time, one forward current pulse followed by one reversecurrent pulse and after that one zero current break.

It has proved that the above method is particularly useful in achievinggood throwing power of copper deposition, i.e., uniform copper layerformation on the outer side of the work piece and on the walls of holescontained therein.

Such objective is however, not achievable with the plating conditionsdescribed, if the substrate to be plated is provided both with regionswhere there are many holes per unit area on the one hand and withregions where there are no or only a few holes per unit area on theother hand. Using the method described in US 2006/0151328 A1 will notconsistently metalize these regions: In those regions, where no or onlya few holes are provided, copper thickness will be large as compared tothose regions which have many holes per unit area.

Furthermore, it has proved disadvantageous that this known method ofhole wall plating leads to differing coppering results in the holes indifferent regions of a board.

It has proved necessary that through holes are first X- (bridge-)plated, i.e., deposition shall lead to enhanced copper deposition in themiddle of the hole thereby closing it by forming a copper plug there,thus forming two blind holes each one being accessible from one of thesides of the board. Then the two hole parts are completely filled whichmeans that the total volume of the hole is filled with metal. When aknown method is used to perform this procedure, holes being located inthe border area of the board will not be plated as efficiently as holesbeing located in the middle thereof. Consequently, the border holes willnot be closed in their center region when the middle holes are filledalready. This leads to an undesirable situation wherein hole-filling isvarying in the different regions of the board.

Furthermore, conformal plating of through holes and blind holes, i.e.,plating of a thin layer of copper on the walls of the holes withoutfilling these, is not uniform when the method of US 2006/0151328 A1 isused.

SUMMARY OF THE INVENTION

Therefore, a first object of the present invention is to provide amethod for electroplating a metal onto a flat substrate, which methodprovides for uniform metal electroplating on work pieces or othersubstrates, more particularly on plate-shaped substrates, such asboards, foils, and the like. More specifically, the method of theinvention shall be suitable to uniformly electroplate a substrate havingboth at least one outer side and holes, i.e., through holes, blindholes, or holes having any other shape, on all surface regions of theseouter sides and inside the holes, i.e., exhibiting a deposit thicknesswhich is as uniform as possible and which does not or only to a minorextent depend on the location on the substrate surface. Still morespecifically, the method of the invention shall be suitable to depositmetal on the outer sides of the board as uniformly as possibleirrespective of whether metal is plated in an area where holes arelocated or in an area where no holes are located. Even morespecifically, the method of the invention shall be suitable to depositmetal in the holes either to uniformly deposit a metal layer on the holewalls (conformal plating) or to uniformly generate a metal plug insidethe holes (X- (bridge-) plating) and subsequently fill the holes withmetal. In this latter case, the method of the invention shall besuitable to fill the holes uniformly irrespective of whether the holesare located near an edge of the board or in the center of the board.Even more specifically, the method of the invention shall be suitable todeposit metal to boards having two sides, either on one side only or onboth sides, wherein either conformal plating or hole filling isperformed.

A second object of the present invention is to provide an apparatuswhich is suitable to perform the method for electroplating the metalonto the substrate according to the invention. The construction,installation, maintenance, and operation of such apparatus shall be aseasy as possible.

The present invention is suitable to achieve the above objects.

The method of the invention comprises the following method steps and maycomprise further method steps:

providing:

the flat substrate, which has two opposing first and second substratesurfaces,

an electroplating apparatus, which comprises at least one counterelectrode (anode); and

an electroplating liquid;

bringing each of the flat substrate with said opposing first and secondsubstrate surfaces and the at least one counter electrode into contactwith the electroplating liquid; and

electrically polarizing said first and second substrate surfaces of thesubstrate to effect metal deposition onto the first and second substratesurfaces by feeding at least one first forward-reverse pulse currentsequence each one being composed of successive first forward-reversepulse periods to the first substrate surface and at least one secondforward-reverse pulse current sequence each one being composed ofsuccessive second forward-reverse pulse periods to the second substratesurface; the first and second forward-reverse pulse current sequencesare applied to the respective substrate surfaces simultaneously;

each one of said at least one first forward-reverse pulse currentsequence at least comprising, in each one of consecutive firstforward-reverse pulse periods, a first forward pulse generating a firstcathodic current during a first forward pulse duration tf1 at the firstsubstrate surface, said first forward pulse having a first forward pulsepeak current if1, and a first reverse pulse generating a first anodiccurrent during a first reverse pulse duration tr1 at the first substratesurface, said first reverse pulse having a first reverse pulse peakcurrent ir1; and each one of said at least one second forward-reversepulse current sequence at least comprising, in each one of consecutivesecond forward-reverse pulse periods, a second forward pulse generatinga second cathodic current during a second forward pulse duration tf2 atthe second substrate surface, said second forward pulse having a secondforward pulse peak current if2, and a second reverse pulse generating asecond anodic current during a second reverse pulse duration tr2 at thesecond substrate surface, said second reverse pulse having a secondreverse pulse peak current ir2;

the first and second forward and reverse pulse peak currents are to beunderstood herein both, as the currents irrespective of the surface areaof the first and second substrate surfaces to which these first andsecond pulse currents are applied, and wherein a current density is thecurrent being applied to a predetermined unit area on the substratesurfaces;

wherein said first and second forward pulses are each further superposedwith a respective first or second superposing cathodic pulse, preferablywith one, or alternatively more than one, superposing cathodic pulse(s),said first and second superposing cathodic pulse(s) having a respectivefirst or second superposing cathodic pulse duration tc1, tc2 which isshorter than the respective first or second forward pulse duration tf1,tf2; and

wherein a phase shift φr between said first reverse pulse of said atleast one first forward-reverse current sequence and said secondsuperposing cathodic pulse of said at least one second forward-reversecurrent sequence is set to 0°±30°.

The apparatus of the invention comprises the following items and maycomprise further items:

means for holding the substrate, wherein the substrate has opposingfirst and second substrate surfaces;

at least one counter electrode (anode);

means for accommodating an electroplating liquid;

means for electrically polarizing the substrate to effect metaldeposition onto the first and second substrate surfaces;

wherein said means for electrically polarizing the first and secondsubstrate surfaces is designed to feed at least one firstforward-reverse pulse current sequence, each one being composed ofsuccessive first forward-reverse pulse periods, to the first substratesurface and at least one second forward-reverse pulse current sequenceeach one being composed of successive second forward-reverse pulseperiods to the second substrate surface;

wherein each one of said at least one first forward-reverse pulsecurrent sequence at least comprises, in each one of consecutive firstforward-reverse pulse periods, a first forward pulse generating a firstcathodic current during a first forward pulse duration tf1 (pulse width)at the first substrate surface, said first forward pulse having a firstforward pulse peak current if1, and a first reverse pulse generating afirst anodic current during a first reverse pulse duration tr1 (pulsewidth) at the first substrate surface, said first reverse pulse having afirst reverse pulse peak current ir1; andeach one of said at least one second forward-reverse pulse currentsequence at least comprises, in each one of consecutive secondforward-reverse pulse periods, a second forward pulse generating asecond cathodic current during a second forward pulse duration tf2 atthe second substrate surface, said second forward pulse having a secondforward pulse peak current if2, and a second reverse pulse generating asecond anodic current during a second reverse pulse duration tr2 at thesecond substrate surface, said second reverse pulse having a secondreverse pulse peak current ir2; andwherein said first and second forward pulses are further superposed witha respective first or second superposing cathodic pulse, preferably withone, or alternatively more than one, superposing cathodic pulse(s), saidfirst and second superposing cathodic pulse(s) having a respective firstor second superposing cathodic pulse duration tc1, tc2 which is shorterthan the respective first or second forward pulse duration tf1, tf2; andwherein said means for electrically polarizing the first and secondsubstrate surfaces is further designed to provide a phase shift φrbetween said first reverse pulse of said at least one firstforward-reverse current sequence and said second superposing cathodicpulse of said at least one second forward-reverse current sequence isset to 0°±30°.

As far as a phase shift between two pulses, for example φr, is mentionedin this description and the claims, it refers to the difference instarting times of the pulses, expressed as an angle being the fractionof a full cycle of 360°.

By using the method and apparatus of the invention, it has proved thatuniform metal plating is achieved on flat substrates. More specifically,especially with copper electroplating, on plate-shaped substrates, suchas boards and foils, metal deposition on the outer sides of thesubstrates (on both sides thereof) is made more uniform even if thesubstrates are provided with first regions which exhibit many holes perunit area and with second regions which exhibit no or only a few holesper unit area. Metal deposition on the outer sides of the substrates inboth regions is made consistent with the method of the invention. Theapparatus of the invention is suitable in performing this method.

For performing the method of the invention the substrate is electricallypolarized to effect metal deposition onto the first and second opposingsubstrate surfaces. To this end, cathodic current pulses and anodiccurrent pulses are generated. Generating these pulses is performed byapplying a voltage across the at least one counter electrode and thesubstrate which are located adjacent to each other. The voltage islikewise created as voltage pulses, i.e., cathodic (forward) voltagepulses to generate cathodic (forward) current pulses and anodic(reverse) voltage pulses to generate anodic (reverse) current pulses.Those skilled in the art will recognize that the voltage and currenttherefore will have an interdependency in that they may be proportionalto each other under the conditions of the method of the invention or atleast a monotonic dependency such that the current rises if the voltageis increased and vice versa. The terms ‘cathodic’ and ‘anodic’ are usedto denote the type of polarization of the substrate: A cathodic(forward) current pulse is of the type of depositing metal on thesubstrate, while an anodic (reverse) current pulse is of the type ofre-dissolving metal from the substrate. To accomplish an overall metaldeposition on the substrate, it will be necessary to shape the cathodicand anodic current pulses such that more metal is deposited than metalis re-dissolved. This will in general be achieved by setting the forwardpulse duration tf longer than the reverse pulse duration tr. It isanyway required that the integral of the forward current pulse (peakcurrent over time) is greater than the integral of the reverse currentpulse (peak current over time). As the reverse pulse peak current it isvery often higher than the forward pulse peak current if, the forwardpulse duration tf must be further extended to accomplish a net (overall)metal deposition.

In general, a rectifier is used to polarize the substrate. The rectifierapplies the pulsed negative or positive potential to the substrate tobring about the respective current pulses. The rectifier on its part maybe controlled by a suitable pulse generator to produce the pulses at therectifier. Furthermore, voltage and current pulses may be generated withany other well-known means to feed a substrate to be electroplated withcurrent pulses.

In principle, the forward pulses, reverse pulses, and superposingcathodic pulses may have any pulse shape. But, a rectangular pulse shapefor any one or any plurality of or all of the forward pulses, reversepulses, and superposing cathodic pulses is preferred. In this respect,it has to be considered that the pulse shapes of the pulses may bedistorted due to a limited pulse raising rate and pulse decaying rate sothat, in principle, a trapezoidal pulse shape (which may, byapproximation, be nearly a rectangular pulse shape) for any one of thesepulses may preferably be applicable.

The above principle is also applicable for superposing said first andsecond forward pulses with the respective first and second superposingcathodic pulses according to the present invention. Superposition willbe achieved by an appropriate control of the voltage/current supply(rectifier), preferably the current supply, thereby generating therespective pulse shape.

Superposition is performed such that the superposing cathodic pulse isshorter than the forward current pulse of the same forward-reverse pulsecurrent sequence. Under this condition, the superposing cathodic pulsecan be located at any time interval during the cathodic current pulse.Consequently, the superposing cathodic pulse may be set to occur at thestart time of the cathodic current pulse, at its time center, or at itsend or at any other point of time during the cathodic current pulse,i.e., it may be set independently with respect to the start time of thecathodic current pulse and with respect to the start time of the reversecurrent pulse which means that ξc (angular offset between the reversepulse and the superposing cathodic pulse within the same forward-reversepulse current sequence) may be set at any value from 0° to 360°. In apreferred embodiment of the present invention, the superposing cathodicpulse is displaced by 180° relative to the reverse pulse, i.e., thestart time of the superposing cathodic pulse is delayed with referenceto the start time of the reverse pulse by 180° (ξc, considering that acomplete cycle of the forward-reverse pulse current sequence covers360°). The superposing cathodic pulse appears as a temporarily raisedcathodic current during the cathodic current pulse. The term‘superposing’ is not to be understood to denote that two currents are tobe superposed to achieve the respective current waveform. The increaseof the current by the superposing cathodic peak current ic during thesuperposing cathodic pulse duration may be achieved in any manner. Theforward pulse peak current if and the superposing cathodic pulse peakcurrent ic add to an overall cathodic peak current ic+f. ic may be setindependently from if (forward pulse peak current) and ir (reverse pulsepeak current) in the same forward-reverse pulse current sequence or indifferent forward-reverse pulse current sequences. Likewise, if may beset independently from ic and ir, and vice versa.

The apparatus of the invention comprises means for holding thesubstrate. The means for holding the substrate may be any holder like aframe which is in turn held by a flight bar for example or may berollers transporting the substrate through a conveyorized apparatus. Themeans for holding the substrate may furthermore be suitable forcontacting the substrate with the electroplating liquid as they causethe substrate to be immersed into a tank for example which contains theelectroplating liquid. If the substrate is electroplated in a so-calledvertical system, i.e., in a plant comprising tanks or containers forholding the electroplating liquid into which the substrate is immersedfor being electroplated, this holding means may be a frame. The framemay be held at the tank or container. If the substrate is treated in aso-called horizontal system, i.e., in a conveyorized plant wherein thesubstrate is conveyed in a horizontal direction while beingelectroplated, the holding means may be conveyorized clamps or rollersor other moving elements clamping or otherwise seizing the substrate.

The apparatus of the invention furthermore comprises means foraccommodating the electroplating liquid. This accommodating means may bea tank or container or any other means which is suitable for storing theliquid.

The apparatus of the invention furthermore comprises means for bringingeach of the substrate and the at least one counter electrode intocontact with the electroplating liquid. If the substrate is treated in avertical system, the substrate contacting means may be a transportcarriage which transports the substrate from one tank or container toanother one and lowers and immerses the substrate into theelectroplating liquid in the tank or container of concern. If thesubstrate is treated in a horizontal system, this contacting means maybe an electroplating liquid delivering means, such as nozzles, or theconveying means which transports the substrate from one conveyorizedmodule of the system to another one if the substrate is immersed by thistransport means into the electroplating liquid in the module of concern.The counter electrode contacting means may be a container of thevertical or horizontal system holding the electroplating liquid whereinthe counter electrode is immersed into.

The apparatus of the invention furthermore comprises at least onecounter electrode, which is required to cause an electrochemicalreaction to occur at the substrate. The at least one counter electrodeis preferably located in the vicinity of the substrate and is, togetherwith the substrate, brought into contact with the electroplating liquid,to cause an electrical current flow between the substrate and the atleast one counter electrode. In a horizontal conveyorized system aplurality of counter electrodes may be placed in succession along theconveyor path for the substrate, either on one side of the conveyor pathor on both sides of the conveyor path.

The apparatus furthermore comprises a means for electrically polarizingthe substrate to effect metal deposition onto the first and secondsubstrate surfaces. This polarizing means serves for feeding electricalenergy to the substrate. For this purpose it may be a current/voltagesource/supply, such as a rectifier. The polarizing means is electricallyconnected to the substrate and the at least one counter electrode.

The polarizing means is furthermore designed to feed the at least onefirst and second forward-reverse pulse current sequences to the at leastone substrate surface. For this purpose the polarizing means iselectrically connected to the substrate surfaces individually and may beequipped with a control means which provides for the generation of theforward-reverse pulse current sequences. Such control means may be anelectrical circuit arrangement which may be driven by a microcontrollerwhich in turn may be programmed by a computer.

The substrate is a flat substrate having opposing first and secondsubstrate surfaces. The first and second substrate surfaces are eachelectrically polarized to effect metal deposition thereon, preferablyindependently from one another. This is achieved by feeding the at leastone first forward-reverse pulse current sequence, each one beingcomposed of successive first forward-reverse pulse periods, each one ofsaid first forward-reverse pulse current sequences having, in each firstforward-reverse pulse period, the first forward pulse, the first reversepulse, and the first superposing cathodic pulse, to the first substratesurface, and the at least one second forward-reverse pulse currentsequence, each one being composed of successive second forward-reversepulse periods, each one of said second forward-reverse pulse currentsequences having, in each second forward-reverse pulse period, thesecond forward pulse, the second reverse pulse, and the secondsuperposing cathodic pulse, to the second substrate surface. The atleast one first forward-reverse pulse current sequence and the at leastone second forward-reverse pulse current sequence are appliedsimultaneously to the substrate surfaces. The two pulse currentsequences preferably have the same frequency and same pulse train, i.e.,same consecutions of pulses. Even more preferably, the at least onefirst and second forward-reverse pulse current sequences may be offsetto each other by a phase shift φs of about 180° (±30°) or of exactly180° (i.e., the phase shift between first and second forward-reversepulse current sequences being defined as the shift between the starttimes of the reverse pulses of the first and second pulse currentsequences, respectively, wherein a complete cycle (first or secondforward-reverse pulse period) covers 360°). A phase shift φs beingexactly 180° means that the start time of the first superposing cathodicpulse on one side of the substrate is at the same time as the start timeof the second reverse pulse on the other side of the substrate, if thesuperposing cathodic pulse and the reverse pulse within the same (firstor second) forward-reverse pulse current sequence are offset relative toeach other by ξc=180° (ξc: angular offset between the start time of thereverse pulse and the start time of the superposing cathodic pulsewithin the same forward-reverse pulse current sequence). Or the phaseshift φs may be substantially lower than 180° such as φs=5° or 10° or15° or 20° or 45° or 90° or 135° or it may have any other value, forexample 180°±30°, more preferably 180°±20° and most preferably 180°±10°.This variation may apply both to conformal plating and X- (bridge-)plating.

The phase shift φs being greater than 0° enables enhancing uniformity ofX- (bridge-) plating in through holes, with a 180° phase shift φsoffering maximum plating in the through holes, i.e., by phasing thefirst and second forward-reverse pulse current sequences being appliedto the opposing sides of the flat substrate such that the superposingcathodic pulse of one of these sequences occurs at the same time as thereverse pulse of the other one of these sequences.

The phase shift φs being greater than 0° also enhances uniformity of thethickness of plated metal on the outer surface of the substrateirrespective of whether the metal is plated in a region where thesubstrate has holes or in a region where the substrate has no or only afew holes.

When conformal plating is applied to a flat substrate having opposingfirst and second substrate surfaces and holes (blind holes and/orthrough holes), a thin layer of copper is plated onto the substratesurface and on the walls of the holes (in case of blind holes also onthe bottom of the holes) without filling these. The phase shift φs beinggreater than 0° as described above also enhances uniformity of thethickness of plated metal on the surface of the substrate duringconformal plating. By applying the superposing cathodic pulse it mighthappen that too much metal is plated into the holes, such that the metallayers plated onto the walls of the holes are too thick. In this case itis also of advantage to change the phase shift φs to lie within a rangeof from 180°±20°, more preferably in a range of from 180°±10°. Byapplying a phase shift φs differing from 180°, uniformity of thethickness of plated metal on the outer surface of the substrate is stillenhanced while the thickness of the metal layers plated onto the wallsof holes is slightly decreased and thus is kept within the desiredthickness range. The same effect is achieved if the angular offset ξcdiffers from 180°, preferably when ξc lies within a range of from180°±20°, more preferably in a range of from 180°±10°, or as the phaseshift φr differs from 0°, namely as φr lies within a range of from0°±30°, preferably within a range of from 0°±20°, even more preferablywithin a range of from 0°±10°.

Furthermore, it is preferred that the durations tr1, tr2 of the firstand second reverse pulses are about the same (±50% relative to the firstreverse pulse duration tr1) or are exactly the same.

Accordingly, to define the chronology of the at least one first andsecond forward-reverse pulse current sequences relative to each other,the following parameters and preferred embodiments are to be considered:

The phase shift φs between the first and the second pulse currentsequences is defined as the shifting of the start times of the reversepulses of the two pulse current sequences relative to each other. Thisparameter is set to be 180° preferably.

The phase shift between the reverse pulse of the first forward-reversepulse current sequence and the superposing cathodic pulse of the secondforward-reverse pulse current sequence, applied simultaneously, isdenoted by φr. This parameter is preferably set to be 0°±30°, 0°±20°,0°±10°, more preferably about 0°, even more preferably exactly 0°. Thisparameter is preferably set to be 0°±Δφr, wherein Δφr is 30°, preferablyis 20°, more preferably is 10°, and wherein φr even more preferably isabout 0° and most preferably is exactly 0°.

The angular offset between the reverse pulse and the superposingcathodic pulse within the same (first or second) forward-reverse pulsecurrent sequence is denoted by ξc. This parameter is preferably set tobe about 180° (±30%) or exactly 180°.

Such further embodiment enhances electroplating in the through holes andenables hole filling. If through holes connecting the first and secondsides are provided, formation of a metal layer on the through hole wallsis made very uniform even if the aspect ratio of the through holes ishigh. X- (bridge-) plating is equally well performed to yield excellentresults because metal electroplating is forced to occur in the holes.Especially differences in closing the through holes in their centerareas in various regions of the substrate do not occur.

In a further preferred embodiment of the present invention, in eachpulse current sequence, the durations/widths tr of the first and secondreverse pulses equal the respective durations/widths tc of the first andsecond superposing cathodic pulses on the respective other side of theflat substrate, i.e., the duration/width tr1 of the first reverse pulsepreferably equals the duration/width tc2 of the second superposingcathodic pulse, and the duration/width tr2 of the second reverse pulsepreferably equals the duration/width tc1 of the first superposingcathodic pulse. More preferably, all durations/widths of the firstreverse pulse tr1, the first superposing cathodic pulse tc1, the secondreverse pulse tr2, and the second superposing cathodic pulse tc2 are atleast approximately the same (±20% relative to the duration/width tr1,tr2 of the reverse pulse).

These further preferred embodiments make uniform electroplating on thehole walls possible, irrespective of the location of the holes on thesubstrate, i.e., irrespective of whether the holes are located in thevicinity of the border of the substrate or in a center region of thesubstrate.

In a further preferred embodiment of the present invention, said firstreverse pulse and said second superposing cathodic pulse are appliedsimultaneously and said second reverse pulse and said first superposingcathodic pulse are applied simultaneously.

In a further preferred embodiment of the present invention, none of theat least one first and second forward-reverse pulse current sequences,either in one of said first and second method section periods only or inboth method section periods, comprises any forward-reverse pulse periodwherein the current is set to zero (pulse break). For this embodiment,the at least one forward-reverse pulse current sequence has proved toyield an improved result as to uniformity of metal deposition in thehole filling process. Contrary to previous results using the method ofUS 2006/0151328 A1 for metal deposition on the hole walls, a pulse breakhaving zero current has been discovered not to be advantageous for X-(bridge-) plating and hole filling. Instead of this achievement, settingthe first and second superposing cathodic pulses simultaneously or atleast nearly simultaneously with the reverse pulse at the respectiveopposing side of the substrate and preferably also for the same durationas the respective reverse pulses, i.e., the first superposing cathodicpulse simultaneously or nearly simultaneously with the second reversepulse and the second superposing cathodic pulse simultaneously or nearlysimultaneously with the first reverse pulse, has proved advantageousover setting pulse breaks simultaneously with the respective reversepulses.

It has also been found advantageous to fill holes using a pulse currentsequence wherein no superposing cathodic pulses are used. Preferably, inthis case, no zero current pulses are used either. It may in such casesalso be advantageous to use first and second forward-reverse pulsecurrent sequences wherein the phase shift φs between the first andsecond reverse pulses thereof is greater than 0° and preferably nearlyor exactly 180°.

In a further preferred embodiment of the present invention, the methodfurther comprises, subsequent to performing the first and secondforward-reverse pulse current sequences to the first and secondsubstrate surfaces in accordance with method steps (d) and (e) in afirst method section period, applying, in a second method sectionperiod, at least one further first and second forward-reverse pulsecurrent sequences each one thereof comprising a plurality of consecutiverespective first or second forward-reverse pulse periods, wherein eachone of the consecutive respective first or second forward-reverse pulseperiods comprises a respective first or second forward pulse generatinga cathodic current during a respective first or second forward pulseduration tf1, tf2 at the respective first or second substrate surface,said respective first or second forward pulse having a respective firstor second forward pulse peak current if1, if2, and a respective first orsecond reverse pulse generating a respective first or second anodiccurrent during a respective first or second reverse pulse duration tr1,tr2 at the respective first or second substrate surface, said first andsecond reverse pulses having a respective first or second reverse pulsepeak current ir1, ir2, without superposing the respective first orsecond forward pulses with a respective first or second superposingcathodic pulse.

In the first method section period, on a first side of the flatsubstrate, a first forward-reverse pulse current sequence is applied andon a second side of the flat substrate, a second forward-reverse pulsecurrent sequence is applied. The first and second forward-reverse pulsecurrent sequences comprise, in this first method section period, firstor second forward pulses, respectively, first or second reverse pulses,respectively, and first or second superposing cathodic pulses,respectively. Furthermore, in the second method section period, on afirst side of the substrate a first further forward-reverse pulsecurrent sequence is applied and on a second side of the substrate, asecond further forward-reverse pulse current sequence is applied. Thefirst and second further forward-reverse pulse current sequences in thissecond method section period comprise first or second forward pulses,respectively, and first or second reverse pulses, respectively, but nofirst or second superposing cathodic pulses, respectively. In the secondmethod section period the first and second forward-reverse pulse currentsequences may be offset to each other by a phase shift φs of 180° orless than 180°, as defined above for the first and secondforward-reverse pulse current sequences comprising superposing cathodicpulses.

This further preferred embodiment serves for filling through holes in asubstrate after a plug has been formed in the holes by using an X-(bridge-) plating technique. The first method section period serves tocreate a plug in the center of the through hole by electrodepositing themetal in the center until metal has built up to plug the hole diameter.Thus, two hole sections are created, one being open to one side of thesubstrate and the other one being open to the other side of thesubstrate. The two hole sections each form a blind hole. In the secondmethod section period these two hole sections are filled from the bottomof the respective blind hole to the respective outer side of thesubstrate.

The individual processing conditions indicated as follows will beapplicable to each one of the following pulse current sequenceconditions (if applicable):

electroplating of each one of both substrate surfaces; or

in the case of applying both, one single or two forward-reverse pulsecurrent sequence(s), to X- (bridge-) plating for through hole filling.

As described herein, any pulse that is indicated to be a forward pulseexerts a cathodic current to the substrate, and any pulse that isindicated to be a reverse pulse exerts an anodic current to thesubstrate.

In a further preferred embodiment of the present invention, the forwardpulse duration tf (first and/or second forward pulse duration) is atleast 5 ms, more preferably at least 20 ms, and most preferably at least70 ms. The forward pulse duration is preferably at most 250 ms, morepreferably at most 150 ms, and most preferably at most 80 ms.

The start time tsf of the forward pulse (first and/or second forwardpulse) may be at any time during the cycle time Tp of the pulse periodof the forward-reverse pulse current sequence.

In a further preferred embodiment of the present invention, the reversepulse duration tr (first and/or second reverse pulse duration) is atleast 0.1 ms, more preferably at least 0.2 ms, and most preferably atleast 1 ms. The reverse pulse duration tr is preferably at most 100 ms,more preferably at most 50 ms, and most preferably at most 6 ms.

The start time of the reverse pulse (first and/or second reverse pulse)may be at any time during the cycle time Tp of the pulse period of theforward-reverse pulse current sequence.

In a further preferred embodiment of the present invention, thesuperposing cathodic pulse duration tc (first and/or second superposingcathodic pulse duration) is at least 0.1 ms, more preferably at least0.2 ms, and most preferably at least 1 ms. The superposing cathodicpulse duration tc is preferably at most 100 ms, more preferably at most50 ms, and most preferably at most 6 ms.

The start time tsc of the superposing cathodic pulse (first and/orsecond superposing cathodic pulse) may be at any time during the forwardpulse.

In a further preferred embodiment of the present invention, the angularoffset ξc between the reverse pulse and the superposing cathodic pulseof a forward-reverse pulse current sequence may be any value of from 0°to 180°. It is preferably about 180° or exactly 180°.

In a further preferred embodiment of the present invention, the forwardpulse peak current if [A] (first and/or second forward pulse peakcurrent), expressed as a forward pulse peak current density If [A/dm2],referring to the surface area of the substrate to be plated, is at least0.1 A/dm2, more preferably at least 0.2 A/dm2, and most preferably atleast 0.5 A/dm2. The forward pulse peak current density If [A/dm2] ispreferably at most 50 A/dm2, more preferably at most 25 A/dm2, and mostpreferably at most 15 A/dm2.

In a further preferred embodiment of the present invention, the reversepulse peak current it [A] (first and/or second reverse pulse peakcurrent), expressed as a reverse pulse peak current density Ir [A/dm2],referring to the surface area of the substrate to be plated, is at least0.2 A/dm2, more preferably at least 0.5 A/dm2, and most preferably atleast 1.0 A/dm2. The reverse pulse peak current density Ir is preferablyat most 120 A/dm2, more preferably at most 80 A/dm2, and most preferablyat most 40 A/dm2.

The superposing cathodic pulse peak current ic [A] (first and/or secondsuperposing cathodic pulse peak current), expressed as a superposingcathodic pulse peak current density Ic [A/dm2], referring to the surfacearea of the substrate to be plated, adds to the forward pulse peakcurrent density If during the superposing cathodic pulse duration tc, sothat the peak current (density) during the period of applying thesuperposing cathodic pulse is the sum of the forward pulse andsuperposing cathodic pulse peak currents (current densities). In afurther preferred embodiment of the present invention, the overallcathodic peak current density Ic+f (or overall peak current ic+f)comprising the forward pulse peak current density If plus thesuperposing cathodic pulse peak current density Ic (or the forward pulsepeak current if plus the superposing cathodic pulse peak current ic,respectively) is at least 0.2 A/dm2, more preferably at least 0.5 A/dm2,and most preferably at least 1.0 A/dm2. The overall cathodic pulse peakcurrent density Ic+f is preferably at most 120 A/dm2, more preferably atmost 80 A/dm2, and most preferably at most 40 A/dm2.

In a further preferred embodiment of the present invention, the ratio ofthe forward pulse duration tf (first or second forward pulse duration)to the reverse pulse duration tr (first or second reverse pulseduration, respectively) of the same forward-reverse pulse currentsequence is at least 1. The ratio of the forward pulse duration tf tothe reverse pulse duration is preferably at most 20 and more preferablyat most 5.

In a further preferred embodiment of the present invention, the ratio ofthe forward pulse peak current density If (first or second forward pulsepeak current density) to the reverse pulse peak current density Ir(first or second reverse pulse peak current density, respectively) ofthe same forward-reverse pulse current sequence is at least 0.0125, morepreferably at least 0.05, and most preferably at least 0.125. The ratioof the forward pulse peak current density If to the reverse pulse peakcurrent density Ir is preferably at most 10, more preferably at most 1,and most preferably at most 0.5.

In a further preferred embodiment of the present invention, a thirdpulse is comprised by the at least one first and/or secondforward-reverse pulse current sequence. This third pulse may be aforward (cathodic) or reverse (anodic) pulse. The third pulse durationta is preferably at least 0.1 ms, more preferably at least 0.5 ms, andmost preferably at least 1 ms. The third pulse duration ta is preferablyat most 100 ms, more preferably at most 50 ms, and most preferably atmost 10 ms.

In a preferred embodiment of the present invention, the angular offsetCa between the reverse pulse and the third pulse may be any value offrom 0° to 180°.

The start time tsa of the third pulse may be at any time during thecycle time Tp of the pulse period of the forward-reverse pulse currentsequence.

In a further preferred embodiment of the present invention, the thirdpulse peak current ia [A], expressed as a third pulse peak currentdensity Ia [A/dm2], referring to the surface area of the substrate to beplated, is at least 0.2 A/dm2, more preferably at least 0.5 A/dm2, andmost preferably at least 1.0 A/dm2. The third pulse peak current densityIa is preferably at most 120 A/dm2, more preferably at most 80 A/dm2,and most preferably at most 40 A/dm2.

In a further preferred embodiment of the present invention, the at leastone first and/or second forward-reverse pulse current sequence may (ineach pulse period) also comprise a pulse break (first and/or secondbreak) wherein the current is set to zero. The pulse break duration tb(first and/or second pulse break duration) is preferably at least 0.1ms, more preferably at least 0.5 ms, and most preferably at least 1 ms.The pulse break duration tb is preferably at most 100 ms, morepreferably at most 50 ms, and most preferably at most 10 ms.

In a preferred embodiment of the present invention, the angular offsetξb between the reverse pulse and the pulse break may be any value offrom 0° to 180°. It is preferably about 180° or exactly 180°. The starttime tsb of the pulse break (first and/or second pulse break) may be atany time during the cycle time Tp of the pulse period of theforward-reverse pulse current sequence.

Due to electrical constraints in real systems, the rise and decay ofcurrent or voltage changes do not occur instantaneously but need acertain time. For this reason, each current or voltage rise or decay isaccompanied by a rising slope and a decay slope. This slope may have aslope duration tsl which is preferably as low as possible and may be atleast 0.05 ms, more preferably at least 0.1 ms, and most preferably atleast 0.2 ms. The slope duration tsl is preferably at most 5 ms, morepreferably at most 2 ms, and most preferably at most 1 ms.

In a further preferred embodiment of the present invention, thefrequency f of the repetition of the at least one first and/or secondforward-reverse pulse periods is at least 0.5 Hz, more preferably atleast 1 Hz, and most preferably at least 3 Hz. The frequency f of therepetition of the at least one forward-reverse pulse period ispreferably at most 20 kHz, more preferably at most 10 kHz, and mostpreferably at most 5 kHz. The cycle time Tp is the reciprocal of thefrequency f.

In a further preferred embodiment of the present invention, the metal iscopper. Such metal is preferably used to create the circuitry onelectrical devices. In general, other metal like nickel, tin, lead, oralloys thereof may be electroplated with the method and apparatus of theinvention.

In a further preferred embodiment of the present invention, theelectroplating liquid contains, in addition to a solvent, water forexample, ions of the at least one metal to be deposited as well as atleast one component enhancing electrical conductivity of the liquid. Theliquid may further contain at least one acid/base adjuster and/or atleast one additive influencing the mechanical, electrical, and/or otherproperties of the metal deposit and/or influencing the thicknessdistribution of the metal deposit and/or influencing the platingperformance of the electroplating liquid including its stability againstdecomposition like oxidation or the like. The ions of the at least onemetal may be hydrated ions or complexed ions. The acid/base adjuster maysimply be an acid or a base and/or be a buffer. The component enhancingthe electrical conductivity of the liquid may be a metal salt or acid orbase. The first and second additives may be brighteners, levelers,antioxidants, carriers, and the like.

If the electroplating liquid is a copper electroplating liquid, thesolvent will be, in general, water. The ions of the at least one metalto be deposited may, in general, be divalent copper ions (Cu2+) havingrespective counter ions like sulfate, methanesulfonate, or pyrophosphateor be bound as a complex. The component enhancing electricalconductivity of the liquid and the acid/base adjuster may be sulfuricacid or any other acid like methane sulfonic acid. The additiveinfluencing the mechanical, electrical, and/or other properties of themetal deposit and/or influencing the thickness distribution of the metaldeposit may be polyethylene glycol and/or an organic compound havingsulfur in a low oxidation state like a disulfide compound. Furthermore,this liquid may contain sodium or potassium chloride.

In order to perform conformal plating in holes in a board and/or ontothe outer surface of a board, a plating conformal composition is usedwhich preferably comprises a copper salt, preferably copper sulfate,sulfuric acid, chloride ions, iron(II) and iron(III) ions to form aredox couple, preferably iron(II) and iron(III) sulfate, and platingadditives. The concentration of the copper salt in the conformal platingcomposition is preferably in a range of from about 22 to about 40 gcopper ions per liter. The optimum concentration thereof is preferably25 g copper ions per liter. The concentration of sulfuric acid in theconformal plating composition is preferably in a range of from about 180to about 240 g/l. The optimum concentration thereof is preferably 200g/l. The concentration of the chloride ions in the conformal platingcomposition is preferably in a range of from about 80 to about 120 mg/l.The optimum concentration thereof is preferably 100 mg/l.

In order to perform X- (bridge-) plating in holes in a board, i.e., tocreate a plug inside the holes, and to fill the holes thereafter, an X-(bridge-) plating composition is used which preferably comprises acopper salt, preferably copper sulfate, sulfuric acid, chloride ions,iron(II) and iron(III) ions to form a redox couple, preferably iron(II)and iron(III) sulfate, and plating additives. The concentration of thecopper salt in the X- (bridge-) plating composition is preferably in arange of from about 65 to about 80 g copper ions per liter. The optimumconcentration thereof is preferably 75 g copper ions per liter. Theconcentration of sulfuric acid in the X- (bridge-) plating compositionis preferably in a range of from about 60 to about 80 g/l. The optimumconcentration thereof is preferably 70 g/l. The concentration of thechloride ions in the X- (bridge-) plating composition is preferably in arange of from about 80 to about 120 mg/l. The optimum concentrationthereof is preferably 100 mg/l.

The concentration of the iron(II) ions in either of these platingcompositions will preferably be at least 1 g/l and will more preferablybe in a range of from about 2 to about 25 g/l. The concentration of theiron(III) ions in either of these plating compositions will preferablybe in a range of from about 0.5 to about 30 g/l and will more preferablybe in a range of from about 1 to about 9 g/l. In general, theseconcentrations may be set higher for conformal plating than for X-(bridge-) plating.

The plating additives may preferably be organic additives, which may bebrighteners, levelers, wetting agents, and the like.

In general, sulfur-containing substances may be used as brighteners. Thebrighteners may for example be selected from the group comprising thesodium salt of 3-(benzthiazolyl-2-thio)-propylsulfonic acid, the sodiumsalt of 3-mercaptopropane-1-sulfonic acid, the sodium salt ofethylenedithiodipropyl sulfonic acid, the disodium salt ofbis-(p-sulfophenyl)disulfide, the disodium salt ofbis-(ω-sulfobutyl)disulfide, the disodium salt ofbis-(ω-sulfohydroxypropyl)-disulfide, the disodium salt ofbis-(ω-sulfopropyl)disulfide, the disodium salt ofbis-(ω-sulfo-propyl)sulfide, the disodium salt ofmethyl-(ω-sulfopropyl)disulfide, the disodium salt ofmethyl-(ω-sulfopropyl)trisulfide, the potassium salt ofO-ethyl-dithiocarbonic acid-S-(ω-sulfopropyl)-ester, thioglycolic acid,the disodium salt of thiophosphoricacid-O-ethyl-bis-(ω-sulfopropyl)ester, the trisodium salt ofthiophosphoric acid tris-(ω-sulfopropyl)ester, and further similarcompounds. The concentration of these substances in either one of theplating compositions lies in a range of from about 0.1 to about 100mg/l.

Polymeric nitrogen compounds (such as polyamines or polyamides) ornitrogen-containing sulfur compounds such as thiourea derivatives orlactam alkoxylates such as those described in DE 38 36 521 C2, which isherein incorporated by reference, can be used as leveling agents. Theconcentration of these substances in either one of the platingcompositions lies in a range of from about 0.1 to about 100 mg/l.

The wetting agents are usually oxygen-containing, high molecularcompounds, for example carboxymethylcellulose, nonylphenol polyglycolether, octandiol bis(polyalkylene glycol ether), octanol polyalkyleneglycol ether, oleic acid polyglycol ester, polyethylene glycolpolypropylene glycol copolymerisate, polyethylene glycol, polyethyleneglycol dimethyl ether, polypropylene glycol, polyvinyl alcohol,β-naphthol polyglycol ether, stearic acid polyglycol ester, stearylalcohol polyglycolether, and similar compounds. The wetting agents maybe present in the composition in a concentration in a range of fromabout 0.005 to about 20 g/l, preferably from about 0.01 to about 5 g/l.

In general, the concentrations of levelers, brighteners, and wettingagents are set to a lower value in an X- (bridge-) plating compositionthan in a conformal plating composition.

In a further preferred embodiment of the present invention, thesubstrate is a circuit carrier, a printed circuit board or chip carrierfor example, wherein the circuit carrier has holes therein. The printedcircuit board may be a double-sided board or a multilayer board having aplurality of inner layers which comprise electrical functionalityincluding electrical circuitry therein. The printed circuit board orother circuit carrier typically comprises on the outer sides and on theholes walls a base metal, preferably copper, layer. The holes may have adiameter of as low as 0.2 mm or may be as large as 2 mm, or the diametermay even be smaller or greater. The board thickness and thus hole length(in case of through holes) may be as low as 0.5 mm and as large as 5 mmor the board thickness may even be smaller or greater. The distance(pitch) of holes to each other may be as low as 0.5 mm or as large as 50mm or even smaller or greater. The holes may be arranged in a matrix(cluster) of for example 20 by 20 mm2.

In general, any other substrates may be electroplated with the methodand apparatus of the invention, including complex-shaped substrates likeplastic or metal parts used in the sanitary, furniture, automotive, ormechanical engineering industry for example.

The following figures and examples explain the invention in more detail.These figures and examples exclusively serve the understanding and donot limit the scope of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 shows an apparatus of the invention in a first embodiment in aschematic perspective view;

FIG. 2 shows an apparatus of the invention in a second embodiment in aschematic perspective view;

FIG. 3 shows a forward-reverse pulse current sequence according to thepresent invention being applied to one surface of a flat substrate;

FIG. 4 shows forward-reverse pulse current sequences in a firstembodiment of the invention, a first one of these forward-reverse pulsecurrent sequences being applied to a first side of a flat substrate anda second one of these forward-reverse pulse current sequences beingapplied to a second side of the flat substrate;

FIG. 5 shows forward-reverse pulse current sequences in a secondembodiment of the invention, each one being applied to one of the sidesof the flat substrate;

FIG. 6 shows forward-reverse pulse current sequences having nosuperposing cathodic pulse;

FIG. 7 shows a forward-reverse pulse current sequence in a thirdembodiment of the invention;

FIG. 8 shows a forward-reverse pulse current sequence having nosuperposing cathodic pulse, but having a pulse break;

FIG. 9 shows photographs of coppered through holes obtained with aforward-reverse pulse current sequence having no superposing cathodicpulse, but a pulse break;

FIG. 10 shows photographs of coppered through holes obtained with aforward-reverse pulse current sequence having no superposing cathodicpulse and no pulse break;

FIG. 11 shows photographs of coppered through holes obtained with aforward-reverse pulse current sequence having a superposing cathodicpulse;

FIG. 12 shows diagrams of copper surface thickness variations atdifferent conditions;

FIG. 13 shows a diagram indicating the dependency of the plated copperthickness between the surface of the board and surface of the throughholes, relating to the plated surface copper thickness [%], from theratio of the active surface area on the surface of the board to thesurface area on the through hole walls;

FIG. 14 shows a diagram indicating copper thickness on the board surfacein a hole region and outside the hole region.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Elements having the same function are designated with the same referencesigns in the figures.

The apparatus of the invention may be of a vertical type of treatmentapparatus 100 (FIG. 1) or a horizontal (conveyorized) type of apparatus200 (FIG. 2).

In the vertical type of apparatus 100 (FIG. 1) the substrate P, aprinted circuit board for example, which has a first surface (side) P1and a second surface (side) P2, is vertically immersed into thetreatment liquid L contained in a container 110. The board is providedwith through holes and/or blind holes. The substrate is placed betweentwo counter electrodes 120, 130 (anodes) which are also oriented in avertical direction and which are arranged facing each other: a firstcounter electrode 120 facing the first surface P1 of the board and asecond counter electrode 130 facing the second surface P2 of the board.Both, the board and the counter electrodes are immersed into thetreatment liquid. The board is held by a holding means 140 like a frameor a claw. The counter electrodes may for example be made from expandedmetal like from expanded titanium which is surface coated with a noblemetal. The treatment liquid may be a copper electroplating liquid like asulfuric acid electroplating liquid containing copper sulfate, sulfuricacid, sodium chloride, and organic additives in water. In addition, theapparatus may contain a heating, nozzles for injecting air into theliquid, nozzles for injecting treatment liquid into the container,stirring means, filtering means, and the like (not shown). Each one ofthe counter electrodes and the board are electrically connected to arespective current source like a rectifier. A first counter electrode120 and the board are connected to a first rectifier 150 (represented byits electrical contacts) and a second counter electrode 130 and theboard are connected to a second rectifier 160 (represented by itselectrical contacts). The current sources independently apply pulsecurrents to the counter electrodes and respective surfaces P1, P2 of theboard. Each one of the pulse currents has a defined pulse shape andfrequency.

The horizontal type apparatus 200 (FIG. 2) also comprises a container210 holding the treatment liquid. Two rows of counter electrodes 220,230 (anodes) are arranged one after the other in the conveyancedirection in the container. A space is formed between the rows wherein aboard P having two surfaces (sides) P1, P2 and being provided withthrough holes and/or blind holes, is conveyed through the container on ahorizontal conveying path. The board is conveyed using rollers 240. Therollers transport the board in a horizontal direction (arrow H) throughthe container. The container is preferably flooded with the treatmentliquid L so that the counter electrodes and the board are completelyimmersed in the treatment liquid. In this case too, each one of thecounter electrodes and the board are electrically connected to arespective current source like a rectifier (well-known in the art). Thefirst counter electrodes 220 and the board are connected to a firstrectifier 250 (represented by its electrical contacts) and the secondcounter electrodes 230 and the board are connected to a second rectifier260 (represented by its electrical contacts). The current sourcesindependently apply pulse currents to the counter electrodes andsurfaces P1, P2 of the board. Each one of the pulse currents has adefined pulse shape and frequency.

In a first embodiment of the method of the present invention, the pulseshape of the pulse current applied to the board (or a flat substratehaving any other shape than being board-shaped) is shown in FIG. 3. Thisdiagram shows the current i over time t with cathodic current beingabove the zero current line (0) and anodic current being below the zerocurrent line (0). The pulse current sequence shown represents oneperiodic cycle having a cycle time Tp. A plurality of such cycles(forward-reverse pulse periods) follow each other. In this embodiment aforward pulse having a forward pulse peak current if is applied during aforward pulse duration tf and a reverse pulse having a reverse pulsepeak current it is applied during a reverse pulse duration tr.Furthermore, during the forward pulse duration tf a superposing cathodicpulse having a superposing cathodic pulse duration tc is applied. Thissuperposing cathodic pulse has a superposing cathodic pulse peak currentis which adds to the forward pulse peak current if to yield an overallcathodic peak current ic+f. This pulse current sequence repeatspermanently at a frequency f, so that the period Tp=1/f.

The pulsed current applied to the substrate P is provided by rectifiers150, 160, 250, 260 which are accordingly programmed to provide suchpulse current sequence. This current sequence is applied to thesubstrate and counter electrodes 120, 130, 220, 230 being arrangedopposite this substrate.

As a flat substrate like a board P is used, the two board's surfaces P1,P2 are independently loaded with forward-reverse pulse current sequencesby applying a first forward-reverse pulse current sequence to a firstcounter electrode 120, 220 and a first surface P1 of the board and byapplying a second forward-reverse pulse current sequence to a secondcounter electrode 130, 230 and a second surface P2 of the board. Thefirst forward-reverse pulse current sequence applied to the first sideof the board is shown in the upper graph of FIG. 4, whereas the secondforward-reverse pulse current sequence applied to the second side of theboard is shown in the lower graph of FIG. 4.

The first forward-reverse pulse current sequence comprises a firstforward pulse having a first forward pulse duration tf1 and a firstforward pulse peak current if1 and a first reverse pulse having a firstreverse pulse duration tr1 and a first reverse pulse peak current ir1.Furthermore, there is a first superposing cathodic pulse having a firstsuperposing cathodic pulse duration tc1 and a first superposing cathodicpulse peak current ic1. The first superposing cathodic pulse peakcurrent ic1 adds to the first forward pulse peak current if1 to yield afirst overall cathodic peak current ic+f1. The second forward-reversepulse current sequence comprises a second forward pulse having a secondforward pulse duration tf2 (not shown) and a second forward pulse peakcurrent if2 and a second reverse pulse having a second reverse pulseduration tr2 and a second reverse pulse peak current ir2. Furthermore,there is a second superposing cathodic pulse having a second superposingcathodic pulse duration tc2 and a second superposing cathodic pulse peakcurrent ic2. The second superposing cathodic pulse peak current ic2 addsto the second forward pulse peak current if2 to yield a second overallcathodic peak current ic+f2. Both pulse current sequences are offset bya phase shift φs of 180°, such that the first reverse pulse is offset tothe second reverse pulse by 180°. Furthermore, the first superposingcathodic pulse of the first pulse current sequence and the secondreverse pulse of the second pulse current sequence are appliedsimultaneously and the second superposing cathodic pulse of the secondpulse current sequence and the first reverse pulse of the first pulsecurrent sequence are also applied simultaneously (φr=0°), because thesuperposing cathodic pulse and the reverse pulse within the sameforward-reverse pulse current sequence are offset relative to each otherby an angular offset ξc=180° and because tc1=tr2 and tc2=tr1. As will beshown hereinafter, this type of pulse current treatment is veryadvantageous for X- (bridge-) plating. If tc1 would not be equal to tr2and tc2 would not be equal to tr1, the reverse and superposing cathodicpulses would not completely overlap.

In a further embodiment (FIG. 5), each one of both pulse currentsequences comprises a forward pulse, a reverse pulse, and a superposingcathodic pulse. The angular offset ξc between the superposing cathodicpulse and the reverse pulse in a pulse current sequence is 110°. Thephase shift φs between the first and the second forward-reverse pulsecurrent sequences is less than 180°, 150° for example.

In yet a further embodiment (FIG. 6), each one of both forward-reversepulse current sequences comprises a forward pulse and a reverse pulse,but no superposing cathodic pulse. These forward-reverse pulse currentsequences may be applied in a second method section period, after, in afirst method section period, the forward-reverse pulse current sequenceshaving superposing cathodic pulses (FIG. 4, 5) have been applied toprovide X- (bridge-) plating of through holes, so that thereafter thethrough holes may be filled efficiently. In this case, the phase shiftφs between the reverse pulses of the two forward-reverse pulse currentsequences is preferably 180°.

In a further method embodiment of the present invention a further(third) pulse is applied, in addition to the forward pulse, the reversepulse, and the superposing cathodic pulse. This pulse current sequenceis shown in FIG. 7. Furthermore in this case, the real pulse trackexhibiting a finite time period for the rise from one current level toanother current level is shown. Therefore each pulse has a rise time anda decay time, indicated as a slope, expressed in [A/s]. This slope mayhave a maximum value depending on electrical conditions of the apparatussetup. The respective rise and decay times (slope durations) of thereverse pulse is accordingly shown to be tsl. Taking the start time forthe reverse pulse to be at 0 s, FIG. 7 further shows a couple of furtherparameters, i.e., the start time for the forward pulse tsf, the starttime for the superposing cathodic pulse tsc, and the start time for theadditional (third) pulse tsa.

Example 1

In a setup using a horizontal conveyorized plating apparatus with aplating liquid flow of 15 m3/h such as shown in FIG. 2, copperdeposition was performed to a printed circuit board having throughholes. The board was held in the apparatus with clamps at one clampingedge thereof wherein the clamps also provided electrical contact to thetwo sides of the board. Each one of the two sides were electricallyconnected individually and powered from a respective rectifierindependently with their own forward-reverse pulse current sequences.The rectifiers were driven by respective computer controlled devices togenerate the forward-reverse pulse sequences. The copper plating bathwas a sulfuric acid plating bath containing copper sulfate, sulfuricacid, sodium chloride and commonly used organic additives. The boardswere provided with a thin copper layer all over the outer surface andthe through hole walls. The through holes had a diameter of 0.2 mm and alength (thickness of the board) of 0.8 mm. 800 through holes werearranged in matrices (clusters) in an area of 20 mm by 20 mm with apitch of 0.5 mm. A couple of these matrices were arranged on the boardat various distances to the edge of the board.

Copper deposition was performed to effect X-plating, i.e., depositingcopper in the through holes to generate a plug in the center thereof.Copper deposition was performed by applying a forward-reverse pulsecurrent sequence to each one of the surfaces of the board, wherein thetwo pulse current sequences were phase shifted relative to each other byφs=180°, i.e., the start time of the first reverse pulse was offset by180° relative to the start time of the second reverse pulse.Furthermore, as the angular offset ξc between the superposing cathodicpulses and the reverse pulses in the same first or secondforward-reverse pulse current sequence was 180°, the start time of thefirst superposing cathodic pulse was at the same time as the start timeof the second reverse pulse.

In a first experiment, deposition was performed with conventionalforward-reverse pulse current sequences for both surfaces of the board,each one having, in each pulse sequence cycle (forward-reverse pulseperiod), one forward pulse, one reverse pulse, and one pulse breakduring which no current flows (Plating Condition 1). The first pulsebreak of the first forward-reverse pulse current sequence was applied atthe same time as the second reverse pulse of the second forward-reversepulse current sequence and vice versa. A diagram showing these pulsecurrent sequences is shown in FIG. 8. The first pulse current sequenceis shown in the upper diagram and the second pulse current sequence isshown in the lower diagram. The parameters for these pulse currentsequences are given in Table 1.

In a second experiment, metal deposition was performed with otherconventional forward-reverse pulse current sequences each one having, ineach pulse sequence cycle (forward-reverse pulse period), one forwardpulse and one reverse pulse, but no pulse break (Plating Condition 2). Adiagram showing these pulse current sequences is shown in FIG. 6. Theparameters for these pulse current sequences are given in Table 1.

In a third experiment according to the present invention, metaldeposition was performed with forward-reverse pulse current sequenceseach one having, in each pulse sequence cycle (forward-reverse pulseperiod), one forward pulse, one reverse pulse, and one superposingcathodic pulse (Plating Condition 3). A diagram of such pulse currentsequences is shown in FIG. 4. The parameters for these forward-reversepulse current sequences are given in Table 1.

Results:

With the conventional forward-reverse pulse current sequence having apulse break (first experiment, Plating Condition 1), marked differencesof X-plating in the through holes were observed depending from thelocation of the through holes on the board: The through holes which werepositioned nearest to the clamping edge of the board (Location 1: at 170mm from the edge of the board opposing the clamping edge) were not yetplugged with copper in the center thereof while thickening of the copperlayer in the center of the holes took place to some extent (FIG. 9a ).The through holes located nearer to the edge of the board which wasopposing to the clamping edge (Location 2: at 85 mm from the edge of theboard opposing the clamping edge) experienced even less coppering sothat only little thickening of the copper layer in the center of theholes took place (FIG. 9b ). The through holes located in the vicinityof the edge of the board opposing to the clamping edge (Location 3: at10 mm distance from the edge of the board opposing the clamping edge)did not show much coppering. A plug has not yet formed at all andthickening was almost not observed (FIG. 9c ). Therefore, metaldeposition is differing between the locations markedly.

With the conventional forward-reverse pulse current sequence having nopulse break (second experiment, Plating Condition 2), plug formationtook place more distinctly at least in those holes which were atLocation 1 and at Location 2 (FIGS. 10a, 10b ). The holes located nearthe edge of the board remote from the clamping location (Location 3)showed marked thickening of the copper layer in the center of the holes,but coppering did not result in plug formation (FIG. 10c ). Therefore,marked differences were still observed while plug formation was betterthan with the first experiment.

With the forward-reverse pulse current sequences having a superposingcathodic pulse according to the invention (third experiment, PlatingCondition 3) almost no differences were observed with plug formation inthe center of the holes irrespective of whether the holes were locatedat Location 1, Location 2, or Location 3 (FIG. 11a : Location 1; FIG.11b : Location 2, FIG. 11c : Location 3).

Example 2

Under the setup conditions of Example 1 (horizontal conveyorized platingline) with a plating liquid flow of 9 m3/h another experiment wasperformed showing superior results as regards uniformity of copper onthe surface of a printed circuit board between through holes. Acomparison was made between the copper thickness obtained betweenthrough holes arranged at a high density hole pitch (0.5 mm) and throughholes arranged at a low density hole pitch (2.0 mm). Comparison was alsomade for different current conditions:

Plating Condition 1: DC plating (DC=direct current).

Plating Condition 2: forward-reverse pulse current sequences with pulsebreak (0 A/dm²), but without superposing cathodic pulse, correspondingto a pulse current sequence as is shown in FIG. 8.

Plating Condition 3: forward-reverse pulse current sequences with asuperposing cathodic pulse, but without pulse break, corresponding to apulse current sequence as shown in FIG. 4.

The board parameters were as follows: panel thickness: 0.8 mm; holediameters 0.2 mm and 0.6 mm; hole pitch: 0.5 mm and 2.0 mm; block area(area of hole matrix): 20 mm by 20 mm.

DC current was set to 2 A/dm2 (Plating Condition 1). All other platingparameters are given in Table 2.

Results:

Copper thickness was measured on the surface of the board between thethrough holes and statistically evaluated. The values for thosemeasuring positions where the hole pitch was small (pitch: 0.5 mm; highhole density) and where the hole pitch was large (pitch: 2.0 mm; lowhole density) were determined separately. The results of thesemeasurements are shown in FIG. 12:

FIG. 12a shows the results of copper surface thickness variationobtained with DC plating (2 A/dm2) at low and high hole density areas(“Low” and “High”, resp.), Plating Condition 1.

FIG. 12b shows the results of copper thickness variation obtained withforward-reverse pulse current sequences and with pulse break but withoutsuperposing cathodic pulse, Plating Condition 2. Again, results obtainedat low and high hole density areas (“Low” and “High”, resp.) are shown.

FIG. 12c shows the results of copper thickness variation obtained withforward-reverse pulse current sequences, without pulse break, but withsuperposing cathodic pulse, Plating Condition 3. Again, results obtainedat low and high hole density areas (“Low” and “High”, resp.) are shown.

Large relative variation in copper surface thickness was obtained withthe pulse conditions using a forward-reverse pulse current sequence withpulse break and without superposing cathodic pulse (Plating Condition2). Surface thickness variation is lower if a superposing cathodic pulseis used (Plating Condition 3). DC conditions are shown for comparisononly (Plating Condition 1). DC conditions are not acceptable if an evenmetal thickness is to be achieved on the surface in high and low holedensity areas.

In a further diagram (FIG. 13) a dependency of the plated surface copperthickness variation between the surface of the board (area withoutthrough holes, plain area) and the surface area including through holes,relating to the ratio of the active surface area between the surface ofthe board (area without through holes, plain area) to the real surfacearea in the through hole area region (board surface area plus surfacearea of the through hole walls) is shown for different hole diameters(0.2 mm: indicated by (1); 0.6 mm, indicated by (2)), different holedensities (hole pitches: 0.5 mm, indicated by (2); 2.0 mm, indicated by(1)), and for different plating conditions (Plating Condition 2 of Table2, indicated by ‘x’; Plating Condition 3 of Table 2, indicated by ‘o’).The data given in Table 2 indicated by 1) and 2) correspond tosubstrates with hole diameters and hole densities indicated by (1) and(2), respectively. Accordingly, copper thickness as plated on the outersurface of the board in a region without through holes is compared tocopper thickness as plated on the outer surface of the board in a regioncomprising through holes.

From the diagram it is apparent that a relatively small copper thicknessdifference between the plain area (without through holes) and thesurface area where through holes were located was achieved if PlatingCondition 3 was used. This effect was more pronounced if substrates withlarge holes and a large hole pitch were plated.

Example 3

Under the setup conditions of Example 1 (horizontal conveyorized platingline) with a plating liquid flow of 9 m3/h, another experiment wasperformed showing superior results as regards uniformity of copper onthe surface of a printed circuit board in an area where through holeswere located (thickness measured between through holes) and outside thisarea, i.e., in regions where no through holes were located.

The board parameters were as follows: panel thickness: 1.5 mm; holediameters 0.4 mm and 0.6 mm; hole pitch: 0.2 mm, 0.4 mm, and 0.8 mm;block area (area of hole matrix): 20 mm by 20 mm.

Comparison is also made for different current conditions (the pulseconsecutions and frequencies of all forward-reverse currents hereinbelow were set to be identical):

Plating Condition 1: forward-reverse pulse current sequences on bothsides of the board, each sequence without pulse break and withoutsuperposing cathodic pulse, the first reverse pulse on one side of theboard being offset to the second reverse pulse on the other side thereofby φs=187°.

Plating Condition 2: forward-reverse pulse current sequences on bothsides of the board, each sequence comprising a superposing cathodicpulse, but no pulse break, the first superposing cathodic pulse on oneside of the board being offset to the second reverse pulse on the otherside thereof and vice versa by φr=7°. The phase shift between the firstand second forward-reverse pulse current sequences was set to φs=187°.The angular offsets between the reverse pulses and the superposingcathodic pulses within the first forward-reverse pulse current sequenceand within the second forward-reverse pulse current sequence,respectively, were set to be ξc=180° in each case.

The parameters for the forward-reverse pulse current sequences are givenin Table 3.

Results:

Copper thickness was measured on the surface of the board between thethrough holes on the one hand and in regions outside this area, i.e., inregions where no through holes were located. The data retrieved werestatistically evaluated. The values for those measuring positions wherethrough holes were present on the one hand and where no through holeswere present on the other hand were determined separately. The resultsof these measurements are shown in FIG. 14:

Irrespective of the application of superposing cathodic pulses or not inthe forward-reverse pulse current sequences, copper thickness in thearea where through holes are present increases when through hole pitchincreases. There is no remarkable effect on copper thickness due to thethrough hole diameter.

A remarkable increase of copper thickness was achieved when theforward-reverse pulse current sequences were applied which additionallyincluded a superposing cathodic pulse as compared to when no such pulsewas additionally included into the sequence. This result clearly showsthat the advantageous effect of providing such superposing cathodicpulse in the forward-reverse pulse current sequence is not onlyeffective if the phase shift φs is 180°, but also when it issubstantially higher, like 187° in this case. It has to be noted thatthis favourable effect is achieved by setting φr to be greater than 0°,i.e., 7° in this case.

TABLE 1 Pulse Parameters for Example 1 Plating Condition 1: PlatingCondition 2: Plating Condition 3. Forward-reverse Forward-reverseForward-reverse pulse current pulse current pulse current sequence, withpulse sequence, no pulse sequence, no pulse break, no break, no break,with superposing cathodic superposing cathodic superposing cathodicpulse pulse pulse Forward current density 3.89 3.68 2.78 I_(f) [A/dm²]Forward pulse duration 76 76 76 t_(f) [ms] Reverse current density 40 4040 I_(r) [A/dm²] Reverse pulse duration 4 4 4 t_(r) [ms] Overallcathodic peak ./. ./. 20 current density I_(c+f) [A/dm²] Superposingcathodic ./. ./. 4 pulse duration t_(c) [ms] Superposing cathodic ./../. 36 pulse duration start [ms] Pulse period T_(p) [ms] 80 80 80 Startpulse break [ms] 36 ./. ./. Pulse break duration t_(b) 4 ./. ./.Effective (=average) 1.5 1.5 1.5 current density I_(av) [A/dm²] Phaseshift φ_(s) between 180° 180° 180° current sequences of both sidesPlating time [min] 30 30 30 Phase shift φ_(r) between ./. ./.  0°reverse pulse and superposing cathodic pulse of different currentsequences

TABLE 2 Pulse Parameters for Example 2 Plating Condition 2: PlatingCondition 3: Forward-reverse pulse Forward-reverse pulse currentsequence with current sequence, no pulse break, no pulse break, withsuperposing cathodic superposing cathodic pulse pulse Forward currentdensity 1) 4.44 1) 3.33 I_(f) [A/dm²] 2) 7.74 2) 7.21 Forward pulseduration 1) 76 1) 76 t_(f) [ms] 2) 78 2) 78 Reverse current density 1)40 1) 40 I_(r) [A/dm²] 2) 14 2) 14 Reverse pulse duration 1) 4 1) 4t_(r) [ms] 2) 2 2) 2 Overall cathodic peak 1) ./. 1) 20 current densityI_(c+f) 2) ./. 2) 20 [A/dm²] Superposing cathodic 1) ./. 1) 4 pulseduration t_(c) [ms] 2) ./. 2) 2 Superposing cathodic 1) ./. 1) 36 pulseduration start [ms] 2) ./. 2) 38 Pulse period T_(p) [ms] 1) 80 1) 80 2)80 2) 80 Start pulse break [ms] 1) 36 1) ./. 2) 38 2) ./. Pulse breakduration t_(b) 1) 4 1) ./. 2) 2 2) ./. Effective (=average) 1) 2 1) 2current density I_(av) 2) 7 2) 7 [A/dm²] Phase shift φ_(s) between 1)180° 1) 180° current sequences of 2) 180° 2) 180° both sides Phase shiftφ_(r) between ./. 0° reverse pulse and superposing cathodic pulse ofdifferent current sequences Plating time [min] 36 36

TABLE 3 Pulse Parameters for Example 3 Plating Condition 1: PlatingCondition 2: Forward-reverse pulse Forward-reverse pulse currentsequence, no current sequence, no pulse break, no pulse break, withsuperposing cathodic superposing cathodic pulse pulse Forward currentdensity 7.8 7.3 I_(f) [A/dm²] Forward pulse duration 76 73 t_(f) [ms]Reverse current density 20 20 I_(r) [A/dm²] Reverse pulse duration 3 3t_(r) [ms] Overall cathodic peak ./. 20 current density I_(c+f) [A/dm²]Superposing cathodic ./. 3 pulse duration t_(c) [ms] Superposingcathodic ./. 36.5 pulse duration start [ms] Pulse period T_(p) [ms] 7979 Effective (=average) 6 6 current density I_(av) [A/dm²] Phase shiftφ_(r) between ./. 7° reverse pulse and superposing cathodic pulse ofdifferent current sequences Plating time [min] 26.7 26.7

LIST OF REFERENCE SIGNS

-   100, 200 electroplating apparatus-   110 means for accommodating an electroplating liquid-   120, 220 first counter electrode-   130, 230 second counter electrode-   140 means for holding the substrate-   150 first means for electrically polarizing the substrate, rectifier-   160 second means for electrically polarizing the substrate,    rectifier-   210 means for accommodating an electroplating liquid-   250 first means for electrically polarizing the substrate, rectifier-   260 second means for electrically polarizing the substrate,    rectifier-   f frequency-   H transport direction-   i current-   i_(a) third pulse peak current-   I_(a) third pulse peak current density-   i_(c) superposing cathodic pulse peak current-   I_(c) superposing cathodic pulse peak current density-   i_(C1) first superposing cathodic pulse peak current-   i_(c2) second superposing cathodic pulse peak current-   i_(c+f) overall cathodic peak current-   I_(c+f) overall cathodic peak current density-   i_(c+f1) first overall cathodic peak current-   i_(c+f2) second overall cathodic peak current-   i_(f) forward pulse peak current-   I_(f) forward pulse peak current density-   i_(f1) first forward pulse peak current-   i_(f2) second forward pulse peak current-   i_(r) reverse pulse peak current-   I_(r) reverse pulse peak current density-   i_(r1) first reverse pulse peak current-   i_(r2) second reverse pulse peak current-   L electroplating/treatment liquid-   P (flat) substrate, board-   P₁ first substrate surface-   P₂ second substrate surface-   t time-   t_(a) third pulse duration-   t_(b) pulse break duration-   t_(c) superposing cathodic pulse duration-   t_(c1) first superposing cathodic pulse duration-   t_(c2) second superposing cathodic pulse duration-   t_(f) forward pulse duration-   t_(f1) first forward pulse duration-   t_(f2) second forward pulse duration-   T_(p) cycle time-   t_(r) reverse pulse duration-   t_(r1) first reverse pulse duration-   t_(r2) second reverse pulse duration-   t_(sa) start time of the third pulse-   t_(sb) start time of the pulse break-   t_(sc) start time of the superposing cathodic pulse-   t_(sf) start time of the forward pulse-   t_(sl) slope duration-   ξ_(a) angular offset between the reverse pulse and the third pulse    within the same forward-reverse pulse current sequence-   ξ_(b) angular offset between the reverse pulse and the pulse break    within the same forward-reverse pulse current sequence-   ξ_(c) angular offset between the reverse pulse and the superposing    cathodic pulse within the same forward-reverse pulse current    sequence-   φ_(r) phase shift between reverse pulse of first forward-reverse    pulse current sequence and superposing cathodic pulse of second    forward reverse pulse current sequence-   φ_(s) phase shift between forward-reverse pulse current sequences    (between start times-   of reverse pulses applied to the two opposing sides of the    substrate)

What is claimed is:
 1. An apparatus for electroplating a metal onto aflat substrate having opposing first and second substrate surfaces, saidapparatus comprising: (a) means for holding the substrate; (b) at leastone counter electrode; (c) means for accommodating an electroplatingliquid; (d) means for electrically polarizing said substrate to effectmetal deposition onto said first and second substrate surfaces; whereinsaid means for electrically polarizing said first and second substratesurfaces of said substrate is a rectifier which is programmed to feed atleast one first forward-reverse pulse current sequence each one beingcomposed of successive first forward-reverse pulse periods to said firstsubstrate surface and at least one second forward-reverse pulse currentsequence each one being composed of successive second forward-reversepulse periods to said second substrate surface, wherein each one of saidfirst forward-reverse pulse current sequence at least comprises, in eachone of consecutive first forward-reverse pulse periods, a first forwardpulse generating a first cathodic current during a first forward pulseduration at the first substrate surface, said first forward pulse havinga first forward pulse peak current, and a first reverse pulse generatinga first anodic current during a first reverse pulse duration at thefirst substrate surface, said first reverse pulse having a first reversepulse peak current; and each one of said second forward-reverse pulsecurrent sequence at least comprising, in each one of consecutive secondforward-reverse pulse periods, a second forward pulse generating asecond cathodic current during a second forward pulse duration at thesecond substrate surface, said second forward pulse having a secondforward pulse peak current, and a second reverse pulse generating asecond anodic current during a second reverse pulse duration at thesecond substrate surface, said second reverse pulse having a secondreverse pulse peak current; wherein said first and second forward pulsesare further superposed with a respective first or second superposingcathodic pulse having a respective first or second superposing cathodicpulse duration which is shorter than said respective first or secondforward pulse duration; and wherein a phase shift between said firstreverse pulse of said at least one first forward-reverse currentsequence and said second superposing cathodic pulse of said at least onesecond forward-reverse current sequence is set to 0°±30°.
 2. Theapparatus for electroplating a metal onto a substrate (P) according toclaim 1, wherein at least one first counter electrode is arrangedopposite a first substrate surface and wherein at least one secondcounter electrode is arranged opposite a second substrate surface andwherein said means for electrically polarizing said substrate aredesigned to feed a first forward-reverse pulse current sequence having,in each first forward-reverse pulse period, a first forward pulse, afirst reverse pulse, and a first superposing cathodic pulse, to a firstsubstrate surface and a second forward-reverse pulse current sequencehaving, in each second forward-reverse pulse period, a second forwardpulse, a second reverse pulse, and a second superposing cathodic pulse,to a second substrate surface, wherein said first and secondforward-reverse pulse current sequences are offset to each other by aphase shift of about 180°.
 3. The apparatus for electroplating a metalonto a substrate according to claim 1, wherein said means forelectrically polarizing said substrate are further designed to set saiddurations of said first and second reverse pulses equal to saiddurations of said first and second superposing cathodic pulses and toapply said first reverse pulse and said second superposing cathodicpulse simultaneously and to apply said second reverse pulse and saidfirst superposing cathodic pulse simultaneously.
 4. The apparatus forelectroplating a metal onto a substrate according to claim 1, whereinthe first forward-reverse pulse current sequences and the secondforward-reverse pulse current sequences have the same frequency and samepulse train.
 5. The apparatus for electroplating a metal onto asubstrate according to claim 1, wherein said means for electricallypolarizing is equipped with a control means which provides for thegeneration of the first and second forward-reverse pulse currentsequences, wherein the control means is an electrical circuitarrangement which is driven by a microcontroller which is programmed bya computer in order to provide for the generation of the first andsecond forward-reverse pulse current sequences.
 6. The apparatus forelectroplating a metal onto a substrate according to claim 1, whereinsaid first and second forward-reverse pulse current sequences are offsetto each other by a phase shift of about 180°.
 7. The apparatus forelectroplating a metal onto a substrate according to claim 1, whereinthe durations of said first and second reverse pulses equal therespective durations of said first and second superposing cathodicpulses.
 8. The apparatus for electroplating a metal onto a substrateaccording to claim 1, wherein said first reverse pulse and said secondsuperposing cathodic pulse are applied simultaneously and wherein saidsecond reverse pulse and said first superposing cathodic pulse areapplied simultaneously.
 9. The apparatus for electroplating a metal ontoa substrate according to claim 1, wherein said rectifier is furtherprogrammed to apply at least one further first and secondforward-reverse pulse current sequences subsequent to performing saidfirst and second forward-reverse pulse current sequences in a firstsection period, and wherein each one of said at least one further firstand second forward-reverse pulse current sequences comprises a pluralityof consecutive first or second forward-reverse pulse periods,respectively, wherein each one of said consecutive first and secondforward-reverse pulse periods comprises a respective first or secondforward pulse generating a cathodic current during a respective first orsecond forward pulse duration at the respective first or secondsubstrate surface, said respective first or second forward pulse havinga respective first or second forward pulse peak current, and arespective first or second reverse pulse generating a respective firstor second anodic current during a respective first or second reversepulse duration at the respective first or second substrate surface, saidfirst and second reverse pulses having a respective first or secondreverse pulse peak current, without superposing said respective first orsecond forward pulses with a respective first or second superposingcathodic pulse, in a second section period.
 10. The apparatus forelectroplating a metal onto a substrate according to claim 9, wherein,in said second section period, said first and second forward-reversepulse current sequences are offset to each other by a phase shift ofabout 180°.
 11. The apparatus for electroplating a metal onto asubstrate according to claim 9, wherein none of said first and secondforward-reverse pulse current sequences, either in one of said first andsecond section periods or in both section periods, comprise any sectionperiod wherein current applied to said substrate is set to zero.
 12. Theapparatus for electroplating a metal onto a substrate according to claim1, wherein said metal is copper.
 13. The apparatus for electroplating ametal onto a substrate according to claim 2, wherein the durations ofsaid first and second reverse pulses equal the respective durations ofsaid first and second superposing cathodic pulses.
 14. The apparatus forelectroplating a metal onto a substrate according to claim 2, whereinsaid first reverse pulse and said second superposing cathodic pulse areapplied simultaneously and wherein said second reverse pulse and saidfirst superposing cathodic pulse are applied simultaneously.
 15. Theapparatus for electroplating a metal onto a substrate according to claim13, wherein said first reverse pulse and said second superposingcathodic pulse are applied simultaneously and wherein said secondreverse pulse and said first superposing cathodic pulse are appliedsimultaneously.
 16. The apparatus for electroplating a metal onto asubstrate according to claim 6, wherein said rectifier is furtherprogrammed to apply at least one further first and secondforward-reverse pulse current sequences subsequent to performing saidfirst and second forward-reverse pulse current sequences in a firstsection period, and wherein each one of said at least one further firstand second forward-reverse pulse current sequences comprises a pluralityof consecutive first or second forward-reverse pulse periods,respectively, wherein each one of said consecutive first and secondforward-reverse pulse periods comprises a respective first or secondforward pulse generating a cathodic current during a respective first orsecond forward pulse duration at the respective first or secondsubstrate surface, said respective first or second forward pulse havinga respective first or second forward pulse peak current, and arespective first or second reverse pulse generating a respective firstor second anodic current during a respective first or second reversepulse duration at the respective first or second substrate surface, saidfirst and second reverse pulses having a respective first or secondreverse pulse peak current, without superposing said respective first orsecond forward pulses with a respective first or second superposingcathodic pulse, in a second section period.
 17. The apparatus forelectroplating a metal onto a substrate according to claim 7, whereinsaid rectifier is further programmed to apply at least one further firstand second forward-reverse pulse current sequences subsequent toperforming said first and second forward-reverse pulse current sequencesin a first section period, and wherein each one of said at least onefurther first and second forward-reverse pulse current sequencescomprises a plurality of consecutive first or second forward-reversepulse periods, respectively, wherein each one of said consecutive firstand second forward-reverse pulse periods comprises a respective first orsecond forward pulse generating a cathodic current during a respectivefirst or second forward pulse duration at the respective first or secondsubstrate surface, said respective first or second forward pulse havinga respective first or second forward pulse peak current, and arespective first or second reverse pulse generating a respective firstor second anodic current during a respective first or second reversepulse duration at the respective first or second substrate surface, saidfirst and second reverse pulses having a respective first or secondreverse pulse peak current, without superposing said respective first orsecond forward pulses with a respective first or second superposingcathodic pulse, in a second section period.
 18. The apparatus forelectroplating a metal onto a substrate according to claim 8, whereinsaid rectifier is further programmed to apply at least one further firstand second forward-reverse pulse current sequences subsequent toperforming said first and second forward-reverse pulse current sequencesin a first section period, and wherein each one of said at least onefurther first and second forward-reverse pulse current sequencescomprises a plurality of consecutive first or second forward-reversepulse periods, respectively, wherein each one of said consecutive firstand second forward-reverse pulse periods comprises a respective first orsecond forward pulse generating a cathodic current during a respectivefirst or second forward pulse duration at the respective first or secondsubstrate surface, said respective first or second forward pulse havinga respective first or second forward pulse peak current, and arespective first or second reverse pulse generating a respective firstor second anodic current during a respective first or second reversepulse duration at the respective first or second substrate surface, saidfirst and second reverse pulses having a respective first or secondreverse pulse peak current, without superposing said respective first orsecond forward pulses with a respective first or second superposingcathodic pulse, in a second section period.
 19. The apparatus forelectroplating a metal onto a substrate according to claim 3, whereinsaid rectifier is further programmed to apply at least one further firstand second forward-reverse pulse current sequences subsequent toperforming said first and second forward-reverse pulse current sequencesin a first section period, and wherein each one of said at least onefurther first and second forward-reverse pulse current sequencescomprises a plurality of consecutive first or second forward-reversepulse periods, respectively, wherein each one of said consecutive firstand second forward-reverse pulse periods comprises a respective first orsecond forward pulse generating a cathodic current during a respectivefirst or second forward pulse duration at the respective first or secondsubstrate surface, said respective first or second forward pulse havinga respective first or second forward pulse peak current, and arespective first or second reverse pulse generating a respective firstor second anodic current during a respective first or second reversepulse duration at the respective first or second substrate surface, saidfirst and second reverse pulses having a respective first or secondreverse pulse peak current, without superposing said respective first orsecond forward pulses with a respective first or second superposingcathodic pulse, in a second section period.